Technical Field
The present disclosure relates to an electromagnetic induction position detection sensor.
Description of the Related Art
An electromagnetic induction position detection sensor is formed by a number of loop coils disposed in X-axis and Y-axis directions of coordinate axes on a board. A position detection device having a position detection sensor permits detection of coordinates of a position pointed to by a position pointer in the X-axis and Y-axis directions based on electromagnetic induction action between the position pointer and the position detection sensor.
Conventional position detection sensors were fabricated by etching copper on an epoxy resin board. Recent years have seen use of position detection devices in compact portable terminals. For this reason, position detection sensors have also been required to be more compact and lighter in weight. Therefore, screen-printed position detection sensors using metal paste that are lower in cost than etched ones and that can be volume-produced have come into use.
Metal paste used not only for position detection sensors but also for ordinary circuit boards includes metallic powder and a resin binder (bonding agent). Then, metal paste applied to the circuit board is subjected to a thermal treatment so that conductivity is enhanced by fusing the metal powder to each other.
Heat-resistant polyimide-based resin sheet is used as a board material on which to screen-print a metal paste. Also, a metal, particularly used as a metal powder of metal paste, is silver. A position detection sensor having a board coated with silver paste through screen printing offers high conductivity and can be fabricated by heating at approximately 200° C. to 250° C., relatively low temperatures for a thermal treatment, for approximately 20 to 30 minutes, and, therefore, has become popular.
However, fabricating a position detection sensor using silver paste had the following problems. First, silver paste is expensive, resulting in higher cost than conventional copper etching. Also, in order to achieve downsizing while at the same time maintaining the number of loop coils of the position detection sensor in today's compact portable terminals against the backdrop of demands for more compact terminals, it is necessary to reduce a loop coil width and a spacing between the loop coils to the extent possible. However, if silver paste is used, the coil spacing cannot be reduced due to ion migration phenomenon. This has led to a problem characterized by difficulties in fabricating compact position detection sensors using silver paste.
For this reason, use of copper paste using copper powder that is cheaper and free from ion migration phenomenon has been proposed in place of silver (refer to Patent Document 1 (WO2012/157704)).
As described in Patent Document 1, however, copper powder has a drawback in that it is prone to formation of an oxide film on the surface and its conductivity degrades due to an oxide layer. In the case of copper in particular, conventional temperatures and heating time result in formation of an oxide film. For this reason, forming loop coils on a sheet-shaped board using copper paste requires a reduction process at temperatures beyond 300° C., temperatures higher than for silver paste, to reduce the oxide film of copper powder, a sintering process at higher temperature, and a short-time thermal treatment to provide resistance to formation of an oxide film.
However, if loop coils are formed on a sheet-shaped board made of polyimide-based resin using copper paste under such high temperatures, a binder included in the copper paste shrinks, leading to shrinkage of paste wiring itself made of copper paste. This shrinkage exerts a shrinking force on the sheet-shaped board, causing problems including warping and undulation on the sheet-shaped board depending on the location. In the event of such warping or undulation, the size of the position detection sensor itself changes, rendering it impossible to detect positions with accuracy.
As a method for reducing this warping caused by shrinkage, a method is proposed in Patent Document 2 (Japanese Patent Laid-Open No. Hei 9-55568) that prints copper paste wiring on both front and back faces of a board.